WIP: (waiting !887) Add yellow info box to team settings of a store to inform about a workaround

What does this MR do?

• Add yellow info box to team settings of a store to inform about a workaround for adding missing team members to store chats
• Deleted two unused variables

Any suggestions for a better wording?

How confident are you it won't break things if deployed?

Should work

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))